1. Field of the Invention
This invention relates to the use of extractive distillation to separate mixtures of an alcohol and its corresponding ester. More particularly, this invention relates to the separation of an azeotrope of an alcohol and its corresponding ester by means of extractive distillation with a higher boiling extraction agent. This invention especially relates to the separation of a C.sub.1 -C.sub.4 alkanol and its corresponding acetate or a C.sub.1 -C.sub.2 alkanol and its corresponding propionate by means of extractive distillation. In one of its preferred embodiments, this invention relates to the use of extractive distillation to effect the transesterification of a C.sub.1 -C.sub.4 alkanol and a dissimilar C.sub.1 -C.sub.4 alkyl acetate or a C.sub.1 -C.sub.2 alkanol and a dissimilar C.sub.1 -C.sub.2 alkyl propionate.
2. Description of the Prior Art
A mixture of liquids which exhibits a minimum or maximum boiling point is termed an azeotrope. Two liquids, whose boiling points are similar often form an azeotrope. Fractional distillation will not separate an azeotrope into its component. However, azeotropic distillation can often be employed to resolve the mixture. In this technique, a third component which forms an azeotrope with one of the closely boiling components is added, the mixture is subjected to distillation and this second azeotrope is removed as the overhead or the bottoms thereby effecting separation of the original azeotrope components. This third component, called an azeotroping solvent, is usually separated subsequently from the component with which it forms the azeotrope by conventionally known means, such as decanting, and returned to the distillation apparatus for reuse. Each azeotropic separation presents its own special problems so as to render past experience of little value and future results unpredictable. Thus, the selection of an azeotroping solvent is seldom a simple task. Not only must the azeotroping solvent form an azeotrope with the proper volatility but the components of this azeotrope must be capable of being separated subsequently in highly pure form for either reuse in the process or recovery as a final, saleable, useful product. In addition, the azeotroping solvent should be relatively inexpensive, non-toxic, non-reactive and non-corrosive.
Extractive distillation is another means for separating azeotrope forming components and involves the addition of a high-boiling liquid, known as an extractive solvent to alter the relative volatilities of the components of the azeotrope. Altering the volatilities is necessary to effect a separation because of the azeotrope-forming propensity of the components (for illustrative purposes a two component azeotrope will be discussed). The formation of new azeotropes between the extractive solvent and either of the two components is avoided by selecting an extractive solvent which boils far above the feed components. In addition, any troublesome azeotropes in the feed disappear in the presence of the extractive solvent. This absence of azeotropes and the ability to recover the extractive agent by a simple downstream distillation makes extractive distillation a simpler and more useful process than azeotropic distillation for the separation of azeotropes.
In a continuous distillation system, the extractive solvent, which is less volatile than the feed components, is always introduced into the column at an intermediate point between the fresh feed tray and the top of the column. The precise tray for introduction of the extractive solvent into the column is chosen so as to provide sufficient fractionation in the top portion of the column to reduce the concentration of the extractive solvent to a negligible amount in the overhead product.
The number of possible extractive solvents available for a given separation is usually much larger than for an azeotropic distillation in view of the less severe volatility limitations. The only restrictions regarding volatity of a serious nature are that the extractive solvent should have a boiling point sufficiently above that of the components of the feed to prevent the formation of an azeotrope and that the solvent not have a boiling point so high that the sensible-heat requirements for the recovery and recycle of the solvent would be uneconomical.
In the methanolysis of polyvinyl acetate and ethylene-vinyl acetate copolymers, methyl acetate is an unavoidable concomitant. Attempts to convert the methyl acetate to higher acetates by ester interchange (transesterification) with higher alcohols have been unsuccessful because methyl acetate is the most volatile component of the reaction mixture and in addition it forms an azeotrope with methanol which is 81% acetate. This results in the reaction being driven in the opposite direction from that which is desired if fractional distillation is employed as the separation means.
U.S. Pat. Nos. 1,433,308 of Steffens and 1,491,076 of Burghart disclose an ester interchange process for preparing higher acetates such as amyl acetate, from methyl acetate or ethyl acetate and amyl alcohol because the azeotrope of methanol-methyl acetate or ethanol-ethyl acetate can be distilled off to remove the product alcohol from the mixture so as to carry the reaction beyond the equilibrium stage. U.S. Pat. No. 1,980,711 of Bannister et al. discloses the preparation and recovery of butyl acetate from butanol and acetic acid. The azeotrope of butanol and butyl acetate is separated by employing water in a liquid-liquid extraction to selectively extract the butanol since the acetate is relatively insoluble in water. U.S. Pat. No. 1,770,414 of Martin et al employs two immiscible solvents in a liquid-liquid extraction of the reaction mixture formed when ethyl acetate and butyl alcohol are transesterified. Water and a water-immiscible hydrocarbon are added to the reaction mixture where they act as selective solvents, the hydrocarbon taking up the greater proportion of ethyl acetate and butyl alcohol while the water dissolves the ethyl alcohol. As a result, ethanol is removed from the reaction and the desired butyl acetate is obtained.
U.S. Pat. No. 2,605,216 of Adelson et al employs azeotropic distillation to separate an azeotrope of an unsaturated alcohol and the saturated monocarboxylic acid ester of the alcohol. The azeotrope-forming agent employed is an aromatic hydrocarbon, such as benzene. The azeotrope of the alcohol and the hydrocarbon goes overhead while the hydrocarbon-free ester is removed as the bottoms. U.S. Pat. No. 3,855,078 of Friedrich et al discloses the extractive distillation of vinyl acetate from methanol using benzene or naphthalene derivatives as the extractive solvent. German Pat. No. 1,066,584 relates to the extractive distillation of azeotropic mixtures of aliphatic carboxylates and their corresponding alcohols, such as methyl acrylate-methanol, ethyl acrylate-ethanol and methyl propionate-methanol using dicarboxylates, hydrocarbons, ketones or keton esters, such as diethyl oxalate, tetra and decahydronaphthalene, octane, nonane, decane, acetophenone, anisole and methyl or ethyl acetoacetate as the extractive solvent. None of this prior art suggests the extractive distillation of lower alkanols and their corresponding esters using an aromatic hydrocarbon as the extractive solvent.
It is an object of this invention to transesterify lower acetates or propionates, which form azeotropes with their corresponding alkanols, to yield higher acetates or propionates and to recover the higher acetates or propionates in highly purified form.
It is another object of this invention to extractively distill azeotropic mixtures of alkanols and their corresponding acetates or propionates.